Some mobile cranes include a jib in addition to a boom which can be freely extended and shortened. Such a crane provided with a jib is illustrated in FIG. 32, for example. A conventional mobile crane provided with a jib will be described hereinafter with reference to FIG. 32. The mobile crane includes an upper revolving superstructure 8, which is free to revolve; a lower traveling body 1; and a boom 3, which is freely extended and shortened and is freely raised and lowered, nearly in the center of the upper revolving superstructure 8. Outriggers 2 are provided in front of and behind and on both sides of the lower traveling body 1 in order to maintain the safety of the body of the vehicle during an operation of the crane. When the boom 3 is, for example, a three-stage telescopic boom, the boom 3 comprises a base boom 4, an intermediate boom 5, and a top boom 6.
The mobile crane includes a main hook 10 at the front end of a wire rope 11 for the main hook 10, which is hung from the front end portion of the top boom 6, and lifts loads with the main hook 10. The operation of a winch makes it possible to windup or let out the wire rope 11 or to extend, shorten, raise, lower, and rotate the boom 3 so as to move loads to a predetermined height and position. However, there are cases where a working range of the boom 3 only is so small that the working range needs to be further enlarged depending on the circumstances of the working site where the mobile crane is placed.
In order to achieve the enlargement of the working range, a jib 7 is connected to the front end portion of the top boom 6, whereby an operation by the jib 7 can be conducted. When not being used, the jib 7 is disconnected from the top boom 6 and is stored, for example, on the left side of the base boom 4 as shown in FIG. 32. When being used, the jib 7 is rotated and extended outwardly in front of the top boom 6. Therefore, the jib 7 is rotatably connected to the top boom 6 via connecting pins 35a and 35b, which are provided respectively on the top and the bottom of the left side of the front end portion of the top boom 6. The jib 7 can be rotated around the connecting pins 35a and 35b within the angle range from the left side of the top boom 6 to the front thereof so as to be extended ahead of the top boom 6.
The jib 7 comprises a jib body 40, a jib bracket 30 for connecting the jib body 40 to the front end portion of the top boom 6, and a jib derricking cylinder 50 for raising or lowering the jib body 40 in relation to the jib bracket 30. The jib bracket 30 is rotatably connected to the jib body 40 via a pivot pin 31, which is vertical to the longitudinal axis of the jib body 40 in a horizontal surface. The jib derricking cylinder 50 is rotatably connected to the jib bracket 30 via a pivot pin 51, which is parallel to the pivot pin 31; and the jib derricking cylinder 50 is rotatably connected to the jib body 40 via a pivot pin 52, which is parallel to the pivot pin 31. The jib body 40 is raised or lowered around the pivot pin 31 in relation to the jib bracket 30 by extending or contracting the jib derricking cylinder 50.
A guide sheave 22, for guiding a wire rope 21 for the auxiliary hook 20, is provided on the upper face of the front end portion of the top boom 6. An end sheave 23, for guiding the wire rope 21 from the guide sheave 22 to the auxiliary hook 20, is provided on the front face of the front end portion of the top boom 6. A guide sheave 32, for guiding the wire rope 21 for the auxiliary hook 20 to the front end of the jib body 40, is provided on the upper face of the jib bracket 30. When the jib 7 is not used, the wire rope 21 for the auxiliary hook 20 is wound up and let out by way of the end sheave 23. When loads are lifted by the jib 7, the auxiliary hook 20 is used. Therefore, the wire rope 21 needs to be changed. The wire rope 21 is removed from the end sheave 23. After the jib 7 is extended, the wire rope 21 is guided from the guide sheave 22, at the front end portion of the top boom 6, to the sheave 23, at the front end portion of the jib body 40, by way of the above guide sheave 32 so as to be used when the jib 7 is being utilized.
When not being used, the jib 7 is usually stored on a side face of the base boom 4. When the jib 7 is stored on the left side face of the base boom 4, brackets are provided at the base boom 4 and the jib body 40, respectively, in order to fix the jib body 40.
In FIG. 33, a bracket 46, having engaging pins 46a and 46b, is fixed almost at the central position of the right side of the jib body 40; and a bracket 47b is fixed to the front end portion 40b of the right side of the jib body 40. Moreover, a bracket 45 and a bracket 47a are fixed at predetermined positions, corresponding to the bracket 46 and the bracket 47b, respectively, on the left side of the base boom 4.
Holes in which a securing pin 48 is inserted are respectively provided in the bracket 47a and the bracket 47b. When the jib 7 is stored, both holes are aligned and fixed with the securing pin 48. FIG. 34 is a drawing of the bracket 45 and the bracket 46 seen from an arrow 34 in FIG. 33. The bracket 45 has holes 45a and 45b which are bored through in a longitudinal direction of the base boom 4. The bracket 46 has the engaging pins 46a and 46b which are respectively inserted and engaged in the holes 45a and 45b from ahead of the base boom 4. When the jib 7 is stored, the jib body 40 is fixed to the base boom 4 in the state that the engaging pins 46a and 46b are respectively engaged in the holes 45a and 45b.
FIG. 35 shows a plan view of a connecting portion of the top boom 6 and the jib bracket 30. It will be described in detail below with reference to FIG. 35.
The brackets 27a and 27b are provided on the top and the bottom respectively, of a left side face 6a of the front end portion of the top boom 6; and the brackets 28a and 28b are provided on the top and the bottom, respectively, of a right side face 6b thereof. In the forward direction of the boom 3 in the state that the jib 7 is stored on the left side of the base boom 4, the brackets 33a and 33b are provided on the top and the bottom, respectively, of the right side of the front end portion of the jib bracket 30; and the brackets 34a and 34b are provided, respectively, on the top and the bottom of the left side thereof. A connecting hole is provided in each of the brackets. The brackets 27a, 27b, 28a, and 28b are connected to the brackets 33a, 33b, 34a, and 34b with connecting pins 35a, 35b, 36a, and 36b, respectively. In this embodiment, an axis, which links cores of the connecting pins 35a and 35b, is included in a vertical surface and in addition is in a vertical direction to the longitudinal axis of the boom 3.
A rotating cylinder 55, for rotating the jib 7 nearly 180 degrees to the top boom 6 around the connecting pins 35a and 35b, is provided. A bracket 37 is provided on the right side of the rear end portion (the portion close to the jib body 40) of the jib bracket 30. One end of the rotating cylinder 55 is rotatably attached to the bracket 37 with a pivot pin 38. The other end of the rotating cylinder 55 is attached to one end of a bracket 56. The other end of the bracket 56 is rotatably connected to one end of a link member 57 and one end of a link member 58 with a pivot pin 59. A bracket 25 is provided on the left side face 6a of the front end portion of the top boom 6. The other end of the link member 57 is rotatably connected to the bracket 25 with a pivot pin 26. The other end of the link member 58 is rotatably connected via a pivot pin 39 to a portion on the top of the right side of the front end portion of the jib bracket 30 close to the front portion of the bracket 33a.
The pivot pins 26, 59, and 39, and the connecting pin 35a compose a four node link. When the rotating cylinder 55 is extended, the pivot pin 59 is pushed ahead of the top boom 6 and the four node link operates to enlarge an angle formed by the link member 57 and the link member 58. Following this operation, the pivot pin 26 and the pivot pin 39 receive a force to move away from each other, with the connecting pin 35a as a center. Thus, the jib bracket 30 is rotated around the connecting pin 35a in the forward direction of the top boom 6. When the rotating cylinder 55 is contracted, contrary to the above, the pivot pin 59 is pulled behind the top boom 6 and the four-node link operates to make the angle formed by the link member 57 and the link member 58 smaller. Thus, the jib bracket 30 is rotated around the connecting pin 35a toward a side of the top boom 6.
When the jib 7 is used in the aforesaid structure, the jib 7 is extended in front of the front portion of the top boom 6 and is connected thereto. Thereafter, the wire rope 21 needs to be changed in order to guide the wire rope 21 for the auxiliary hook 20 from the front portion of the top boom 6 to the front portion of the jib body 40. FIG. 36 shows the process of extending the jib 7 and changing the wire rope 21, and FIGS. 37-40 are drawings for explaining this process. Detailed explanation will be made below with reference to FIGS. 36-40. Incidentally, in the following explanation, each step of the process is represented by "S". For example, "S21" represents "step 21".
FIG. 37 shows an initial condition of the process. Here, as preparations for extending the jib 7, the outrigger 2 of the lower traveling body 1 is extended to the maximum extent.
S21: The boom 3 is made nearly horizontal with its shortest length.
S22: The wire rope 21 for the auxiliary hook is winched out and loosened (which is the state of a wire rope 21a in FIG. 37), to make the auxiliary hook 20 touch a ground surface GL and lay down.
S23: The securing pin 48 (See FIG. 33) of the jib 7 is removed and the jib bracket 30 is pushed toward the top boom 6 side by human power or a hydraulic actuator and the like. This operation is called "swing a jib out".
S24: The connecting holes of the brackets 27a and 27b on the left side face 6a of the front end portion of the top boom 6 and the connecting holes of the brackets 33a and 33b at the front right end portion of the jib bracket 30 are matched and connected by inserting the connecting pins 35a and 35b therein, respectively.
S25: The wire rope 21 for the auxiliary hook is removed from the end sheave 23 and hung along the right side of the top boom 6, which is the state of the wire rope 21 in FIG. 37.
S26: The boom 3 is Gradually raised to a predetermined angle. At this time, the boom 3 is raised while the wire rope 21 is loosened so as to keep the auxiliary hook 20 touching the ground surface.
S27: The boom is extended from its shortest condition by a predetermined length.
FIG. 38 shows a side elevational view of a mobile crane in the aforesaid condition. While the boom 3 is raised, the wire rope 21 keeps hanging alone, the right side of the top boom 6 from the guide sheave 22 on the upper face of the front end portion of the top boom 6. Subsequently, when the boom is extended from its shortest condition by the predetermined length, the jib body 40 is pulled upwardly in the forward direction of the boom with the extension of the boom 3, since the front portion of the jib bracket 30 is connected to the top boom 6 via the connecting pins 35a and 35b. Then, the engaging pins 46a and 46b of the jib 7 are removed from the holes 45a and 45b of the bracket 45 (See FIG. 34). In this condition, the jib derricking cylinder 50 is locked in a stopped condition; therefore, the jib body 40 is supported to the jib bracket 30 by the jib derricking cylinder 50.
S28: The jib derricking cylinder 50 is operated to the jib body 40 downwardly so as to be nearly vertical to the longitudinal axis of the boom 3.
FIG. 39 shows the aforesaid condition. The jib body 40 is rotated around the pivot pin 31 by the jib derricking cylinder 50 to the position 40a and is nearly vertical to the longitudinal axis of the boom 3. At this time, the front end portion of the jib 7 gets closer to the ground surface GL, since the boom 3 is raised by the predetermined angle and extended by the predetermined length.
S29: The rotating cylinder 55 is extended to rotate the jib bracket 30 and the jib body 40 by 180 decrees around the connecting pins 35a and 35b so as to extend the jib body 40 in front of the top boom 6 (at the position 40b in FIG. 39).
S30: While the jib derricking cylinder 50 is operated to rotate the jib body 40 around the pivot pin 31 and raise the same, the boom 3 is gradually pivoted downwardly so that the jib body 40 and the boom 3 will be nearly horizontal.
FIG. 40 shows the aforesaid condition. When the jib body 40 and the boom 3 are nearly horizontal, the auxiliary hook 20 is laid down at the same position 20a where the auxiliary hook 20 was placed on the ground surface GL in the initial step of the process, that is, near the front end portion of the top boom 6.
S31: The holes of the brackets 28a and 28b on the right side face 6b of the front end portion of the top boom 6 and the holes of the brackets 34a and 34b of the jib bracket 30 are aligned and connected by inserting the connecting pins 36a and 36b therein, respectively (See FIG. 35).
S32: The auxiliary hook 20 and the wire rope 21 are moved to be near to the front end portion of the jib 7. The wire rope 21 is inserted into the guide sheave 32 on the upper face of the jib bracket 30, a load sheave 41, and a guide sheave 42 at the front end portion of the jib body 40.
At this time, as shown in FIG. 40, an operator carries the auxiliary hook 20 to a position 20b close to the front end portion of the jib 7. Moreover, the operator is also required to carry the wire rope 21 for changing the same.
As described above, in a conventional method for extending a jib, after the jib 7 is extended, operators are required to carry the auxiliary hook 20 from the vicinity of the front end portion of the boom 3 to the vicinity of the front end portion of the jib 7. As the operators must carry the heavy auxiliary hook 20, there occur disadvantages in that the operability is low and the operators' fatigue becomes larger.
In addition, when the wire rope 21 is changed, since the wire rope 21 is heavy, the operators move the wire rope 21 while letting the wire rope 21 touch the ground surface during the movement. Therefore, the wire rope 21 becomes dirty, which easily causes a reduction in the life of the wire rope 21, abrasion of other portions, or the like. When the wire rope 21 is let out while the auxiliary hook 20 is placed on the ground surface, a winch can be unwound without applying tension to the wire rope 21, whereby disordered winding occurs early when the wire rope 21 is wound up on a winch drum.
In addition, after the jib 7 is extended, the wire rope 21 needs to be changed by inserting it into the guide sheave 32 of the above frame body, the guide sheave 42 of the jib 7, and the load sheave 41 of the jib 7. This operation of changing the rope is conducted at a high spot for which a stepladder is necessary, so that it is dangerous and a very laborious operation for operators. This operation is a heavy operation in that operators directly touch the wire rope 21 with their hands. Therefore, their hands become smeared with oil or dirt from the wire rope 21, and this operation may cause injuries. Furthermore, this operation takes a lot of time, thereby causing a poor operating efficiency and a heavy burden for operators. When the jib 7 is swung outwardly and one side of the jib bracket 30 and one side of the top boom 6 are connected, the alignment of the connecting holes for connecting them is difficult. Even after the connecting holes are aligned, operators are required to insert the pins. Accordingly, this operation needs a person's assistance.